The prevalence of fiber lasers in the medical, military and industrial fields has increased the overall demand for high-power fiber lasers. As the number of fiber lasers needed by various industries increases, the manufacturers of fiber lasers requires improved techniques to efficiently manufacturing such high-power fiber lasers to meet the increasing demand.
A number of fiber lasers are known in the prior art, for example U.S. Pat. No. 9,360,625 B2, International Pub. No. WO 2015/102697 A2 and U.S. Pat. Pub. No. 2016/0099538 A1. Also known in the prior art are methods and devices for splicing optical fibers such as, for example U.S. Pat. No. 6,612,754.
A typical prior art laser is illustrated in FIG. 1 comprises of a number of individual fiber based components. In the illustrated example, the assembled fiber laser 2 comprises six individual components which are all coupled to one another in a desired configuration and arrangement. The components are arranged, from a pump end 4 of the fiber laser 2 to a terminus end 6 thereof, in the following order: a laser diode module 10, a high-reflector fiber Bragg grating 12, a rare-earth doped fiber 14; a partial-reflector fiber Bragg grating 16, a cladding mode stripper 18 and a fiber cable connector 19. As each of these components as well as their associated purpose and functions are generally well known in the art, further details concerning the same are not provided or believed be necessary.
As is conventional art, these components are manufactured or fabricated individually, i.e., independent of one another. Laser diode modules can include injection laser diodes and optically pumped semiconductor lasers. These diode modules are electrically or optically pumped semiconductor lasers that can control the flow of electrical or light energy, convert the electrical energy to light and amplify or increase the light energy before emitting the light energy via an un-doped passive optical fiber.
A fiber Bragg grating is typically fabricated by optically machining an un-doped passive optical fiber. This optically machining procedure involves illuminating the core material of the fiber, which in turn induces some structural changes in the fiber and thus modifies the refractive index of the fiber. Fiber Bragg gratings are used to alter input light in the optical fiber by reflecting/refracting differing ranges of wavelengths of the input light. For example, a high-reflector fiber Bragg grating allows for a greater amount, e.g., 99% of the input light to pass through the high-reflector fiber Bragg grating, i.e., only a small percentage of the wavelengths of the input light are prevented from passing through a high-reflector Bragg grating. In contrast, a partial-reflector fiber Bragg grating allows a smaller amount, e.g., 80% of the input light, to pass through the partial-reflector fiber Bragg grating. A partial-reflector fiber Bragg grating increases percentage of the wavelengths of the input light that are prevented from passing through the partial-reflector fiber Bragg grating.
Rare-earth doped fibers are formed from un-doped passive optical fiber which are doped with laser-active rare earth ions such as neodymium, ytterbium, erbium and thulium to name a few. These ions absorb light which excites them into metastable levels. This facilitates amplification of the light, which was input into the doped fiber, by stimulated emission.
Generally the term cladding mode stripper refers to conventional methods and means which cause light propagating in the cladding (i.e., not the core in which waveguide modes are desired) to propagate out of the cladding. To achieve this, the cladding mode stripper absorbs or redirects the stripped light. Cladding mode strippers in high-power fiber amplifiers are made from a double-clad fiber. The differences in the properties or characteristics of the core and the clads of the fiber facilitate the redirection and adsorption of stripped light.
Fiber connectors are generally used as the terminations of optical fiber cables and provide nonpermanent connections between fiber-coupled devices. Fiber connectors generally include a floating ferrule, into which an un-doped fiber is inserted.
The above described components are each either conventionally coupled to or formed with distinct individual fibers. That is, the laser diode module 10 is formed with optical fiber 20. In addition, the high-reflector fiber Bragg grating 12 is also conventionally formed with another optical fiber 22. Further, the rare-earth doped fiber 14 is formed with a still further fiber 24. Next, the partial-reflector fiber Bragg grating 16 is conventionally formed with an optical fiber 26. The cladding mode stripper 18 is then formed with still another optical fiber 28. Lastly, the fiber cable connector 19 is formed with a further fiber 29. These components are formed with or utilize the same type of optical fiber at the start of their production. The individual fibers used in the fabrication of the different components are typically double-clad, passive optical fibers which have a polymer cladding that provides a multimode waveguide for the pump energy.
In order to assemble the fiber laser 2 illustrated in FIG. 1, a fiber end of a first component to a fiber end of an adjacent second component and then splicing a fiber end of a third component to the opposite fiber end of the second component and so on until all the components are spliced together end to end in the desired order. The fiber ends of the components are spliced together by means of fusion splices. The process of fusion splicing the ends of the fibers of two adjacent components generally includes the steps of: stripping the fibers, cleaning the fibers, cleaving the fibers, splicing the fibers, testing the connection and protecting the connection.
In more detail, the splicing process begins by preparing the fiber ends of at least two components to be fused together. This requires that all of the protective coatings on the ends of the fibers of the at least two components be removed or rather stripped. Then the bare fiber ends are cleaned with an alcohol and wipes, being careful to ensure that moisture is not attracted to the bare fiber ends. Subsequently, the bare fiber ends are cleaved, using a “score-and-break” method to ensure that the faces of the fiber ends are perfectly flat and perpendicular to the axis of the fibers. Then, the two cleaved fibers are automatically aligned in the x, y, z planes by means of a fusion splicer so that, depending on the fusion splicer being utilized, the cores or the cladding of the fibers are aligned. Once aligned, the fiber ends of the adjacent components are heated and melted by a heating element and then fused together so as to connect the fibers end to end. The splices are then evaluated for example to ensure that the completed splice is strong enough to withstand damage during shipping, handling and extended use. Finally, the bare area of the fused fibers is protected by re-coating the area of the fibers with something such as a heat shrinkable membrane.
Although, splicing is an acceptable means for coupling the fibers of different components, it is recognized that connections of fibers that are made by fusion splicing have a number of potential drawbacks.
It is known for example that, if the surface of the fiber at or near the splice becomes compromised in any manner, the mechanical strength of the splice and its surroundings may be below that of the normal bare fiber. Such damage can include surface scratches which can be caused during the removal of the protective coating at the ends of the fiber. Damage can also be caused after splicing such as during the later application of a protective coating. In addition, faulty fusion splicing of fibers is known to negatively impact the transmission of light energy through fiber lasers thus reducing the effectiveness, efficiency and durability of the components and thus the fiber laser. Further, equipment for fusion splicing is fairly expensive, and may require extensive training.
Fiberoptic devices and systems ubiquitously employ fused splices to join fibers. As noted, each splice presents opportunities for optical and mechanical degradation of the device. Additionally, every splice takes time, equipment and resources to execute and packaging strategies that account for service loops and splice placement. The quality, cost and size of fiberoptic devices can also depend heavily on the total number of splices in the device. In FIG. 1 each splice is represented by an individual circle which connects the fibers of two adjacent components. The fiber laser 2 comprises six different components each having their own fiber. The fiber laser 2 thus requires a total of five splices 30, 32, 34, 36, 38 to connect the laser diode module 10, the high-reflector fiber Bragg grating 12, the rare-earth doped fiber 14; the partial-reflector fiber Bragg grating 16, the cladding mode stripper 18, and fiber cable connector 19. There is a strong motivation to reduce the number of splices required to fully assemble the fiber laser 2, as this would increase product quality and reliability and significantly reduce manufacturing costs. Considering the generally high cost of laser materials (mainly doped gain media and pump energy sources) it is advantageous to minimize touch labor on those materials so as to mitigate labor-based and poor-product-quality costs. For high-power fiber lasers, eliminating optical interfaces such as those caused by splices is critical as splices are sources for degradation in reliability, decreased performance and increased manufacturing costs.